engine light problem
 

I am a newbie and would appreciate any help with my problem.
I have a S5 and when I am driving the car, the check engine light will come on
and the tachometer will drop to zero. A few minutes later, the engine light will
go off and the tachometer will come back on. The car sems to run ok when this
happens with no misfiring or loss of power.
I have been told that I cannot check the computer code on the car unless the
check engine light is on.
Any suggestions as to what my problem could be? Thank you in advance for
your time.

http://www.johnr.com/cpucodes.html

This information is also in the 2nd gen FAQ at the top of this section.

Thank you for the information.
The mechanic tells me that he does not have the plug to fit the connector to obtain the error codes. He says that he thinks it is the crank angle sensor but I hate to throw money at a problem if it doesn't fix it. I live in a small community with a limited amount of mechanics, none of which are very familiar with RX7s.
Has anyone ever heard of my check engine light and tachometer problem happening and what caused it.
Thank you in advance for any help.

If you have an S5 then just ground the Orange wire at the Green check connector located by the leading coil (this Green plug only has one wire and yes, it's Orange) and w/key to on the error codes should be displayed.

Post #8 and look to the right of the larger Green diagnostic plug for the small Green plug w/one Orange wire.

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/help-identifying-switched-power-source-under-hood-998013/

Talked to the mechanic and he was able to hook up the connector and it didn't show any error codes. He checked the resistance on the crank angle sensor and showed 162.8 G and 161.2 nc.
Any ideas as to what could cause the check engine light to come on and tach to quit? Is an error code the only thing that will give me a reason? It seems to do it when the engine is warm.
Thanks for your help as most of this is Greek to me.

A loose fuse (Engine fuse, EGI INJ fuse), wires connected to those fuses, faulty Main Relay.